The present invention relates to power control in radio communications, and more particularly, to more efficient and effective open loop power control. The adaptive open loop power control approach of the present invention is particularly well-suited for communications in a Code Division Multiple Access (CDMA) cellular radio system.
Power control is very important in radio communications systems, and particularly so in third generation Wideband Code Division Multiple Access (WCDMA) cellular systems. Before transmitting over an xe2x80x9cuplinkxe2x80x9d channel, a mobile station must set its transmission power level. Similarly, the radio access network must set base station transmit power on xe2x80x9cdownlink channels,xe2x80x9d e.g., a paging channel (PCH), a forward access channel (FACH), in addition to traffic channels (TCH. Indeed, the actual power level set for mobile and base station radio transmission and the interference levels that result therefrom are significant concerns in all mobile radio communications systems.
The physical characteristics of a radio channel may vary significantly due to a number of reasons. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. If the transmission power of a mobile station signal is too low, the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of the signals adds to the delay associated with radio access procedures, increases data processing overhead, and reduces the available radio bandwidth because signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system.
Interference is a particularly severe problem in CDMA systems where a large number of radios transmit and receive on the same frequency. If one mobile station transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of signals received from other mobile radios to the point that a receiving base station cannot correctly demodulate transmissions from the other mobile radios. In fact, if a mobile station transmits a signal at twice the power level needed for the signal to be accurately received at the base station receiver, that mobile signal occupies roughly twice the system capacity as it would if the signal were transmit at the optimum power level. Unregulated, it is not uncommon for a strong mobile station to transmit signals that are received at the base station at many, many times the strength of other mobile transmissions. The loss of system capacity to such excessively xe2x80x9cstrongxe2x80x9d mobile stations is unacceptable.
Additional problems are associated with transmitting with too much power. One is the so-called xe2x80x9cparty effect.xe2x80x9d If a mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can xe2x80x9cbe heardxe2x80x9d compounding the already serious interference problem. Another problem is wasted battery power. It is very important to conserve the limited battery life in mobile radios. By far, the largest drain on a mobile""s battery occurs during transmission. A significant objective for any power control approach, therefore, is to reduce transmit power where possible without increasing the number of retransmissions to an unacceptably high level as a consequence of that power reduction. Except for battery consumption, the above-described problems with setting transmission power also apply to downlink radio transmissions from base stations.
There are two basic approaches to power control: open loop and closed loop. In open loop power control, the transmit power is calculated at the transmitter based on one or more parameters, and the calculated value is used to set the transmit power level. In particular, the transmit power is adjusted in order to match an estimated path loss so that the signal is received at the base station at a predetermined power level. Closed loop power control relies on feedback from the receiver so that the transmitter knows, for example, at what power level (and sometimes also at what interference level) the transmitted signal was received. Using this feedback, the transmitter then appropriately adjusts its transmit power level. Alternatively, the receiver may simply order the transmitter to increase or decrease its transmit power. The additionally received feedback information means that closed loop power control is generally more accurate than open loop power control.
Common channels for both uplink and downlink shared by several mobile radios are typically used to transmit relatively short control signaling messages which do not justify the additional xe2x80x9ccostxe2x80x9d in terms of delay, signaling overhead, spreading code allocation, and bandwidth consumption associated with dedicated channels. Common channels may also be utilized to transmit short traffic data packets appended directly to the typical control messages sent on common channels. xe2x80x9cLower costxe2x80x9d open loop power control is well-suited for transmission over common channels being faster, less complicated, and occupying fewer radio resources than closed loop power control commonly used for dedicated channels.
One type of common channel shared by mobile stations is a random access channel which provides communication between plural mobile stations and a base station when those mobile stations have not been allocated a dedicated channel. Access channel messages may include for example call reservations, responses to pages, orders, registrations, and small size user data packets. However, because multiple mobile stations may be using the random access channel at the same time, each additional mobile station transmitting on that access channel contributes to the background noise and interference thereby diminishing the system""s finite capacity. Consequently, it is important to set the appropriate output power of the mobile station before transmitting.
Thus, before performing a random access, the mobile station caculates an open loop transmission power Ptx to be used on the random access channel in the uplink direction so that the mobile""s signal is received at the base station at a predetermined power level. In particular, the mobile station strives to achieve a target Carrier-to-Interference Ratio (CIR) xcex3t at the base station. The carrier-to-interference ratio actually received at the base station corresponds to the received uplink carrier power CUL minus the uplink interference IUL. The received carrier power CUL corresponds to the mobile""s transmit power level Ptx minus the path loss L. The open loop power control therefore can determine the transmit power {circumflex over (P)}tx as a function of the target carrier-to-interference ratio xcex3t, an uplink interference estimate ÎUL, and a path loss estimate L. The path loss estimate {circumflex over (L)} may be obtained with the mobile station measuring the received power of a known signal (e.g., a downlink pilot or other broadcasted signals) transmitted on a downlink channel by the base station. The known signal includes a message that informs the mobile of the power at which that known signal is being transmit by the base station. The uplink interference is estimated (measured) and broadcast by the base station over the cell together with a downlink pilot signal output power value. The transmit power {circumflex over (P)}tx may then be determined using the target CIR xcex3t, the uplink interference estimate ÎUL, and the path loss estimate {circumflex over (L)} in accordance with the following open loop power control algorithm:
{circumflex over (P)}tx=xcex3t+ÎUL+{circumflex over (L)}xe2x80x83xe2x80x83(1)
Unfortunately, the open loop power control algorithm in equation (1) suffers from uncertainties that make the received carrier-to-interference ratio different (often significantly different) from the target carrier-to-interference ratio xcex3t. For example, the estimated path loss {circumflex over (L)} typically differs from the actual path loss L because of a number of factors such as: (1) the actual power at which the base station transmits the pilot signal likely differs from the broadcasted downlink pilot signal output power value, (2) inaccurate measurements of signal strength in the mobile station, and (3) fading, noise, and delays in measuring the path loss. Similarly, the uplink interference IUL can change significantly since it was last measured by the base station. And even if the transmit power {circumflex over (P)}tx determined by the open loop power control procedure is a reasonably accurate estimate, the actual transmit power Ptx at which the mobile station transmits likely differs from the commanded transmission power due to imperfections and hardware limitations in the mobile station implementation. For example, the mobile transmit power varies significantly depending on the current temperature of the mobile station and on the non-linearity of components employed in the mobile station. The actual received carrier-to-interference ratio xcex3 at the base station will be
xcex3=Ptxxe2x88x92IULxe2x88x92Lxe2x80x83xe2x80x83(2)
The actual transmit power Ptx, the actual uplink interference, and the actual path loss cannot be known with certainty. The shortcomings of open loop power control described for uplink transmission also apply to open loop power control for downlink transmission on common channels, e.g. the forward access channel (FACH), etc.
These various factors may cause the received carrier-to-interference ratio xcex3 to differ from the target carrier-to-interference ratio xcex3t by as much as xc2x110 dB or more. FIG. 1 illustrates this uncertainty through a sketch of the probability density function (PDF) ƒxcex93(xcex3) of the received CIR. In reality, the probability density function of the received CIR, probably would likely be closer to a Gaussian distribution. FIG. 1 simply illustrates the fact that there is a spread of the received CIR.
In summary, the limitations of the above-described open loop power control approach and the practicalities of operation (e.g., temperature) and implementation (e.g., non-linear components) make it difficult to achieve an appropriate open loop transmit power for the current transmission under the current circumstances. The end result is either a failed communication (too low of a transmit power) or unnecessary interference with resulting system capacity loss (too high of a transmit power).
One way to address some of the above-described problems is to employ power xe2x80x9crampingxe2x80x9d such as described in Ericsson""s U.S. Pat. No. 5,430,760 to Dent. The mobile station initiates a random access at a low initial transmit power level and gradually (e.g., incrementally) increases the transmission power level until the base station detects and acknowledges the access signal. Once detected, power level of the message is maintained at the detected level. One drawback with this power ramp-up approach is that it could introduce significant delay into the access procedure. Specifically, there may be substantial delay between access attempts while the mobile waits for an acknowledgement of the most recently transmitted access signal. This delay is particularly undesirable at low traffic load levels when interference caused by a random access transmission is of less importance. On the other hand, if the ramp-up occurs too quickly, it would likely reach too high of a power level by the time the detected signal is acknowledged. Another drawback is the need for an acknowledgment from the receiver. The acknowledgment message could be lost or not received with result that the ramp-up continues unnecessarily. At a minimum, the acknowledgment adds some complexity.
To decrease the delay in the power ramping procedure, power ramping on preamble level may be employed as described in commonly-assigned U.S. patent application Ser. No. 09/166,679, filed Oct. 5, 1998, and incorporated herein by reference. The mobile station transmits only short preamble signals with increasing power until the base station detects the received preamble energy (as opposed to decoding the entire random access message) and sends back a positive acquisition indicator to the mobile. However, regardless of how power ramping is effected, the initial transmit power level from which to start ramping must be decided by the transmitter based on the above described open loop power control.
It is an object of the present invention to achieve an optimal power control method that accounts for current transmission conditions.
It is an object of the present invention to provide an adaptive power control technique that ensures a satisfactory quality of communication at a minimal level of interference.
It is an object of the present invention to determine a compensated transmit power level that results in an actually received CIR that is at or close to a target CIR.
It is an object of the present invention to reduce transmit power without increasing the number of retransmissions as a consequence of that power reduction.
It is an object of the present invention to extend the battery life of mobile stations by controlling the transmit power level of the mobile station to a minimal but still effective transmit power level.
It is an object of the present invention to eliminate unnecessary delays in radio access when open loop power control is employed in an access transmission, especially during low traffic conditions.
It is an object of the present invention to eliminate unnecessary delays in radio access when preamble power ramping is employed in an access transmission especially during low traffic conditions.
It is an object of the present invention to provide a flexible mobile station power control technique that takes into account current interference level(s), mobile station specific parameters, and other factors without requiring a power detection acknowledgment signal or other power-related feedback from the receiver.
It is an object of the present invention for the mobile station to adapt an open loop power setting by recognizing a power level feedback received in the acknowledge message from the base station both at successful and unsuccessful random accesses.
It is an object of the present invention for the mobile station to adapt and compensate for the temperature, mobile station systematic errors, and base station systematic errors.
The adaptive power control of the present invention overcomes the above-identified problems and satisfies these and other objects. In a preferred, example embodiment, delay on a common radio communications channel employed by plural mobile stations to communicate with a base station located in a corresponding geographical cell area is minimized by adapting transmission power based on a traffic load. For lower traffic loads, a higher transmission power is permitted. For higher traffic loads, a lower power level is set.
Transmit power level is determined using a desired signal ratio, such as a target CIR, a transmission path loss over the radio communications channel, and an interference value. An adaptive power parameter is also introduced which adapts the transmit power control based on one or more other current communications conditions and/or characteristics of the mobile station. For example, the adaptive power parameter may be a function of a current interference in a base station cell either alone or in combination with a current interference in one or more neighboring cells. The adaptive power parameter may also account for a type of data packet connection to be employed between the mobile station and the base station after random access, a mobile station""s subscription, a current temperature of the mobile station, a current base station used by the mobile station, a current estimated path loss between the mobile station and base station, and/or other factors.